The invention relates to a method for the quantitative determination of the fissile material content in small size particles which are present in samples such as environmental samples.
In order to detect non-declared nuclear activities in nuclear or non-nuclear installations environmental samples inside and/or outside suspected installations must be taken and analyzed in order to detect their fissile material content beyond or below the natural enrichment rate. The isotopic ratio U-235/U-238 which in nature amounts to 0.7:99.3 is particularly significative for such verifications.
Usually this ratio is measured by means of a mass spectrometry process. However, this method is cumbersome, requires a clean laboratory for the preparation of the environmental samples in view of the analysis and destroys the sample.
Therefore there is a need for a quick and cheap screening method since many of the environmental samples do not contain any fissile material besides natural uranium and thorium. A pre-screening of the environmental samples in order to discard from further analysis all samples without fissile material and samples containing only natural uranium and thorium would allow a severe reduction of the mass spectrometry operations.
In the frame of uranium mining a method has been conceived which allows the estimation of the natural uranium content in rock samples or in solutions and which is known as xe2x80x9cSolid State Fission Trackxe2x80x9d process. This method is for example described in the periodical NUCLEAR INSTRUMENTS AND METHODS 109 (1973) pages 453 to 459. According to this method a rock sample to be analyzed is sandwiched between sheets of certain organic materials and submitted to a defined thermal neutron flux. Since among the uranium isotopes only U-235 is fissionable by thermal neutrons, fission tracks created in the sheets by fission products reveal the presence of U-235. The track density found on these sheets depends on the content of U-235 in the sample.
It is an aim of the present invention to propose a method of the type defined above which allows a quick and cheap screening of the environmental samples on single particles of suspect material. This reduces considerably the number of mass spectrometry operations.
This method is defined in the appended claim 1.
Preferably a standard chart is used for the interpretation of the tracks, this chart containing, in a columns and lines arrangement, images of fission tracks from standard particles of stepped known size and of different stepped known enrichment ratios. Since the number of tracks (or track density) depends on the size of the particle and on its enrichment ratio, tracks created by a particular particle emerge reveal the identification of said particle.
At first, its size is measured by means of an optical or electron microscope. Further, the density of tracks associated to this particle is compared for that given size with the corresponding density of an image of the chart. The resulting type of fissile material and its enrichment ratio can be read directly from the chart.